1. Field of the Invention
The present invention is related to a control circuit, especially to a control circuit with deep burst mode for the power converter.
2. Description of the Related Art
A variety of power converters have been widely used to provide regulated voltage and current for various electronic products. Based on the restriction of environmental pollution, power converters have been required to meet the power management and energy conservation standards. The principle of power management is to manage the power consumption of system during its operation. Further, only a little quantity of power will be consumed during non-operation. With respect to the power converters in a power management application, saving power in a light-load condition is a major requirement.
Generally, a control circuit of the power converter generates a switching signal in response to a feedback signal. The feedback signal is correlated to a load condition of the power converter. The switching signal is used to switch a power transformer of the power converter for regulating an output of the power converter. In order to reduce the power consumption of the power converter, the power converter performs a burst mode for regulating the output of the power converter when the load condition of the power converter is light-load.
FIG. 1 shows waveforms of the conventional power converter performing the burst mode. Once the feedback signal VFB is lower than a threshold VOZ, the switching signal VG is disabled for reducing the output of the power converter.
In other words, the power consumption is reduced when the load condition is the light-load. Further, once the feedback signal VFB is larger than a threshold VOZR, the switching signal VG is enabled and then disabled after a short on-time, as like an impulse signal. In other words, the switching signal VG is switched to generate the impulse signal. The impulse signal is used to switch the power transformer of the power converter for providing a little quantity of power.
As shown in FIG. 1, when the load condition of the power converter is the light-load and the feedback signal VFB is larger than the threshold VOZR, the switching signal VG is continuously switched until the feedback signal VFB is lower than the threshold VOZ. Therefore, the switching signal VG is switched many times. Once the feedback signal VFB is larger than the threshold VOZR again, the switching signal VG is continuously switched until the feedback signal VFB is lower than the threshold VOZ, again. Therefore, during each burst period T2, the switching signal VG is switched many times, so the power transformer is also switched many times during each burst period T2. It causes the increment of the switching loss, and therefore the power consumption can't be reduced efficiently. Generally, the burst period T2 and the burst frequency are constant. For example, the burst frequency is 22K Hz. T1 shown in FIG. 1 is a switching period of the switching signal VG under the burst mode.
Accordingly, the present invention provides a control circuit with deep burst mode to reduce the switching loss for reducing the power consumption.